58 The Birth and Death of the Sun

the box with the plates. But when he developed his photo-
graphs he found that the plates were badly spoiled, as if
they had been previously exposed to light. This was very
strange, since the plates had been carefully wrapped in
thick black paper and never yet opened. The only object
in the drawer that might have been responsible for the
damage was the preparation of uranium bisulphate, which
had for so long rested so close to the plates.

Is it possible, thought Becquerel, turning over in his
hands the ampoule with the suspected material, that this
substance, spontaneously and without any previous excita-
tion, emits some invisible, highly penetrating radiation
that can pass without difficulty through the cover of the
box and the black paper and affect the photographic emul-
sion? To answer this question, he repeated the experiment
with some new plates. But this time he deliberately placed
an iron key from one of the drawers between the photo-
graphic plate and the hypothetical source of the mysteri-
ous radiation.

A few days later, Becquerel's hands were probably shak-
ing with excitement as, under the red lamp of the photo-
graphic darkroom, a diffuse silhouette of the key began to
appear slowly against the darkening background of the
negative. Yes, it definitely was a new kind of radiation
coming from the atoms of uranium., a radiation that easily
penetrated materials nontransparent to ordinary light but
was still unable to pass through the thickness of an iron
key!

Subsequent investigations have shown that the only
other element known at that time capable of the same type
of spontaneous radiation was thorium) the heaviest element
after uranium; but the laborious search undertaken by a